Urea is generally produced by the well known method of contacting NH.sub.3 and CO.sub.2 to form ammonium carbonate and of dehydrating the latter to urea. The first reaction is instantaneous and substantially complete; the second one is much slower and incomplete, and it takes place only in the liquid phase. It is also known that in the presence of excess NH.sub.3 the conversion of ammonium carbamate to urea is promoted, and that in the presence of excess water it is hindered.
The formation of ammonium carbamate is strongly exothermic, and the dehydration of ammonium carbamate to urea is endothermic, but to a lesser degree. For this reason, generally excess heat must be removed from the urea synthesis reactor if the formation of ammonium carbamate and the dehydration of carbamate to urea are simultaneously carried out in the same vessel.
The excess exothermic heat of reaction is usually removed from the urea synthesis reactor by producing steam in a coil immersed in the urea synthesis mixture. The common drawbacks of such a method of removal of the excess heat of reaction from a urea synthesis reactor are as follows: 1. a relatively large reactor coil is usually required due to the small temperature differential normally available between the synthesis reactor mixture and the boiling condensate in the reactor coil. This problem becomes very much pronounced in the case that steam must be produced in the reactor coil at a relatively high and usable level and at the same time the reactor is operated at a relatively high NH.sub.3 to CO.sub.2 reactor feed mol ratio, for instance above about 3.4 to one. It is a well known fact that, in the presence of excess NH.sub.3 in the urea synthesis reaction mixture, the vapor pressure of the reaction mixture is increased and its boiling point is decreased, thus requiring a lower reactor operating temperature when operating at a predetermined and constant reactor pressure level. 2. Local overheating of the urea synthesis reaction mixture occurs due to poor heat transfer rate to the coil, with consequent vaporizing of the reactants NH.sub.3 and CO.sub.2, and consequent loss in conversion of ammonium carbamate to urea.
There are two specific cases in which the reactor coil usually is not required because of the fact that the excess exothermic heat available in the urea synthesis reactor is absorbed by a relatively large amount of either excess NH.sub.3 or carbamate recycle solution or both, fed to the urea synthesis reactor at a relatively lower temperature. For example, in the so called "ONCE-THROUGH Urea Synthesis Processes", the unconverted reactants present in the reactor effluent are not recycled back to the reactor, but they are separated as gas from the aqueous urea product solution by steam heating at reduced pressure and are sent to an adjacent plant for recovery and for the production of either ammonium sulfate or ammonium nitrate. In such Once Through Processes the amount of Liquid NH.sub.3 reactor feed can be increased in practice to the point at which all the excess exothermic heat available in the urea synthesis reactor is used internally for the heating of the excess liquid NH.sub.3 reactor feed stream to the reactor operating temperature inside the reactor. Customarily, in this case the liquid NH.sub.3 reactor feed and the reactor operating temperature are maintained, respectively, at about 20.degree. C. and about 180.degree.-185.degree. C.
Furthermore, in the so called "Partial or Total Carbamate Solution Recycle Urea Synthesis Processes", the above-mentioned unconverted reactants separated from the aqueous urea product solution in the form of a gaseous mixture, instead of being used for the production of ammonium sulfate or nitrate, are absorbed in water to form an ammoniacal aqueous solution of ammonium carbamate, and are recycled into the urea synthesis reactor partially or totally, at a usual temperature of about 90.degree.-100.degree. C. In this latter case, the excess exothermic heat available in the reactor is used internally to elevate the temperature of the carbamate solution recycle stream from 90.degree.-100.degree. C. to the above-mentioned reactor operating temperature of 180.degree.-185.degree. C. Obviously in such a case, if the amount of recycled carbamate solution is relatively large, the corresponding amount of heat in deficiency must be added to the reactor if one wishes to maintain its operating temperature at a certain optimum and desired temperature level. This amount of heat in deficiency is usually added to the reactor by preheating the relatively cold liquid NH.sub.3 reactor feed stream from the above-mentioned temperature of about 20.degree. C. to 80.degree. C. There is a common drawback to both such specific cases with respect to the conversion in the reactor, as will be explained below.
As discussed above, it becomes evident that in both cases, either in a Once Through or in a Carbamate Solution Recycle urea synthesis process, the exothermic heat of reaction available in the urea synthesis reactor is wasted by being used internally for the sole purpose of bringing the relatively colder reactor feed streams up to the operating temperature at which the urea synthesis reactor is maintained. Therefore, such a reactor runs completely adiabatically, without heat removal or heat addition, once the reactor feed streams are introduced into the reactor. However, if the relatively colder reactor feed streams are preheated beyond the point at which a urea synthesis reactor operates adiabatically, as for instance in accordance with the method described in U.S. Pat. No. 3,579,636, it becomes necessary to remove from the reactor the equivalent amount of heat added to the reactor feed streams in excess of the normal requirement of an adiabatic reactor. In such a case, the urea synthesis reactor becomes exothermic again, as for instance in the case of the above mentioned Once Through urea synthesis processes.
As mentioned above, the problem of removing heat from the urea synthesis reactor becomes more complicated when excess NH.sub.3 is used in the reactor for the purpose of attaining a higher degree of conversion of ammonium carbamate to urea.
It has been found that by removing the excess heat of reaction, available in an exothermic reactor, in an external high pressure heat exchanger substantially operating at the same reactor pressure and at a relatively lower NH.sub.3 to CO.sub.2 molar ratio than the reactor, and that by subsequently contacting the resulting reaction mixture with additional excess NH.sub.3 in a substantially adiabatic urea synthesis reactor, considerable advantages are attained, as described further below.